Uh oh!
There was an error while loading. Please reload this page.
bpo-40791: Make compare_digest more constant-time. - #20444
Merged
Conversation
The existing volatile `left`/`right` pointers guarantee that the reads will all occur, but does not guarantee that they will be _used_. So a compiler can still short-circuit the loop, saving e.g. the overhead of doing the xors and especially the overhead of the data dependency between `result` and the reads. That would change performance depending on where the first unequal byte occurs. This change removes that optimization. (This is change python#1 from https://bugs.python.org/issue40791 .)
gpshead
approved these changes
May 27, 2020
gpshead
commented
May 27, 2020
Member
Can you add a NEWS entry (https://blurb-it.herokuapp.com/ can do it for you)? |
ssbr
commented
May 28, 2020
ContributorAuthor
OK, wasn't sure if this would deserve a news entry. Used the blurb tool -- really nice! Thanks for the recommendation. :) (Feel free to tweak the exact wording of the news entry, committers should have edit access to my PR if I understand it right.) |
ned-deily
commented
Jun 16, 2020
Member
miss-islington
commented
Nov 21, 2020
Contributor
miss-islington pushed a commit
to miss-islington/cpython
that referenced
this pull request
Nov 21, 2020
* bpo-40791: Make compare_digest more constant-time. The existing volatile `left`/`right` pointers guarantee that the reads will all occur, but does not guarantee that they will be _used_. So a compiler can still short-circuit the loop, saving e.g. the overhead of doing the xors and especially the overhead of the data dependency between `result` and the reads. That would change performance depending on where the first unequal byte occurs. This change removes that optimization. (This is change GH-1 from https://bugs.python.org/issue40791 .) (cherry picked from commit 3172936) Co-authored-by: Devin Jeanpierre <jeanpierreda@google.com>
bedevere-bot
commented
Nov 21, 2020
GH-23436 is a backport of this pull request to the 3.9 branch. |
miss-islington pushed a commit
to miss-islington/cpython
that referenced
this pull request
Nov 21, 2020
* bpo-40791: Make compare_digest more constant-time. The existing volatile `left`/`right` pointers guarantee that the reads will all occur, but does not guarantee that they will be _used_. So a compiler can still short-circuit the loop, saving e.g. the overhead of doing the xors and especially the overhead of the data dependency between `result` and the reads. That would change performance depending on where the first unequal byte occurs. This change removes that optimization. (This is change GH-1 from https://bugs.python.org/issue40791 .) (cherry picked from commit 3172936) Co-authored-by: Devin Jeanpierre <jeanpierreda@google.com>
bedevere-bot
commented
Nov 21, 2020
GH-23437 is a backport of this pull request to the 3.8 branch. |
bedevere-bot
commented
Nov 21, 2020
GH-23438 is a backport of this pull request to the 3.7 branch. |
miss-islington pushed a commit
to miss-islington/cpython
that referenced
this pull request
Nov 21, 2020
* bpo-40791: Make compare_digest more constant-time. The existing volatile `left`/`right` pointers guarantee that the reads will all occur, but does not guarantee that they will be _used_. So a compiler can still short-circuit the loop, saving e.g. the overhead of doing the xors and especially the overhead of the data dependency between `result` and the reads. That would change performance depending on where the first unequal byte occurs. This change removes that optimization. (This is change GH-1 from https://bugs.python.org/issue40791 .) (cherry picked from commit 3172936) Co-authored-by: Devin Jeanpierre <jeanpierreda@google.com>
miss-islington added a commit
that referenced
this pull request
Nov 21, 2020
* bpo-40791: Make compare_digest more constant-time. The existing volatile `left`/`right` pointers guarantee that the reads will all occur, but does not guarantee that they will be _used_. So a compiler can still short-circuit the loop, saving e.g. the overhead of doing the xors and especially the overhead of the data dependency between `result` and the reads. That would change performance depending on where the first unequal byte occurs. This change removes that optimization. (This is change GH-1 from https://bugs.python.org/issue40791 .) (cherry picked from commit 3172936) Co-authored-by: Devin Jeanpierre <jeanpierreda@google.com>
miss-islington added a commit
that referenced
this pull request
Nov 21, 2020
* bpo-40791: Make compare_digest more constant-time. The existing volatile `left`/`right` pointers guarantee that the reads will all occur, but does not guarantee that they will be _used_. So a compiler can still short-circuit the loop, saving e.g. the overhead of doing the xors and especially the overhead of the data dependency between `result` and the reads. That would change performance depending on where the first unequal byte occurs. This change removes that optimization. (This is change GH-1 from https://bugs.python.org/issue40791 .) (cherry picked from commit 3172936) Co-authored-by: Devin Jeanpierre <jeanpierreda@google.com>
adorilson pushed a commit
to adorilson/cpython
that referenced
this pull request
Mar 13, 2021
* bpo-40791: Make compare_digest more constant-time. The existing volatile `left`/`right` pointers guarantee that the reads will all occur, but does not guarantee that they will be _used_. So a compiler can still short-circuit the loop, saving e.g. the overhead of doing the xors and especially the overhead of the data dependency between `result` and the reads. That would change performance depending on where the first unequal byte occurs. This change removes that optimization. (This is change #1 from https://bugs.python.org/issue40791 .)
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for freeto join this conversation on GitHub.
Already have an account?
Sign in to comment
7 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
The existing volatile
left/rightpointers guarantee that the reads will all occur, but does not guarantee that they will be used. So a compiler can still short-circuit the loop, saving e.g. the overhead of doing the xors and especially the overhead of the data dependency betweenresultand the reads. That would change performance depending on where the first unequal byte occurs. This change removes that optimization.(This is change 1 from https://bugs.python.org/issue40791 .)
https://bugs.python.org/issue40791